1. Field of the Invention
The present invention relates to the field of rotor blades for wind turbine installations. In particular, it relates to means for extending a chord-wise dimension of a portion of said rotor blades.
2. Description of the Related Art
Wind turbine installations are continuously being developed to enable the installation to capture and subsequently convert an increasing amount of the energy represented by the wind into electricity. In particular, it is desirable to increase the surface area of the blade that is presented to the wind to enable a more efficient capture of said energy. However, in providing a rotor blade having an increased surface area, increased loading is experienced by the structure of the blade.
Blade design involves optimisation of a number of characteristics of the blade. This optimisation typically involves selection of the aerofoil section to be used and variation of the aerofoil section along the span-wise length of the blade, camber of the blade and twist of the blade along a span-wise length. A rotor blade is varied in geometry in the span-wise direction, as the speed of the blade through the air increases with distance from the rotor hub. Furthermore, as the distance from the rotor hub increases, the air becomes “cleaner”, in other words, there is less interference from other bodies such as the rotor hub itself and other, adjacent, blades.
In order to achieve optimal design for the blade in a root region of the blade, i.e. a proximal end of the blade, it is desirable to extend the chord-wise dimension to compensate for slower tangential velocity in this region. However an increase in dimension of the blade can cause structural problems.
FIG. 1 shows one type of a conventional rotor blade 2 comprising a load bearing, spar member 4 extending substantially the length of the blade, to which is connected an outer surface 6 of the blade 2. This outer surface is, generally, smoothly configured to enable air (or other fluid) to pass over in a streamlined manner. Rotor blades experience significant structural loading in operation, not only due to the aerodynamic loads exerted thereon but also due to the magnitude and weight of the structure of the rotor blade itself. These loads are primarily transmitted to the spar member 4 and from there to a hub (not shown) of the wind turbine.
In operation, the rotor blades 2 of a wind turbine rotate through a substantially vertically orientated plane. Consequently, significant cyclic loading is experienced by each blade. In particular, fluctuating tensile and compressive loads are experienced along a foremost or “leading” edge 8 of the blade 2 and along a rearmost or “trailing” edge 10 of the blade 2. Hereinafter, these particular loads are referred to as “edge-wise loads”. The edge-wise loads are most significant in a root region of the rotor blade 2, for example for the 30% of the blade nearest to a hub of the wind turbine (once installed).
Whilst the edge-wise loads are experienced by both the leading edge 8 and the trailing edge 10, the trailing edge is located further from the neutral axis of the rotor blade 2 and therefore higher strains are experienced at the trailing edge 10 of the rotor blade. Furthermore, by locally increasing the chord-wise dimension in a root region of the rotor blade 2 (as depicted in FIG. 1), the trailing edge 10 describes a convex profile when viewed in plan form. It follows that when edge-wise loads are experienced along this profile, the material bounded by the trailing edge 10 is also exposed to the increased, fluctuating strain. In particular, a difficult to resist chord-wise load is exerted on the material effectively compressing the trailing edge 10 tending to cause this material to buckle.
In some rotor blades the cross section varies from representing an aerofoil at a region of maximum chord dimension to becoming circular in cross section at a root of the rotor blade. Such a variation means that the curvature described by the trailing edge 10 (when viewed in plan form) is more extreme. As the curvature is more extreme, the fluctuating strains experienced by the material bounded by the trailing edge are correspondingly increased.
It is, therefore, desirable to provide a means for increasing the chord of the blade, in a localised manner to enhance the aerodynamic performance of the rotor blade, whilst minimising a corresponding increase in structural loading.